1. FIELD OF THE INVENTION
The present invention relates to a pneumatic multi-stage actuator usable to actuate a movable machine part such as a damper incorporated in an automotive air conditioner, for example.
2. DESCRIPTION OF THE PRIOR ART
An example of the pneumatic multi-stage actuator of the class specified above is disclosed in Japanese Patent Publication No. 49-6075 (6075/1974) published for opposition purpose on Feb. 12, 1974. The actuator disclosed in the Japanese publication comprises a hollow housing formed by two cup-shaped housing members formed of sheet metal. Two diaphragms of an elastomeric material are disposed in the housing to divide the interior thereof into three chambers one of which is vented to atmosphere, the two other chambers being pneumatically connected through conduits and valve means to a vacuum source. Compression coil springs are disposed in the vacuum chambers to act against the force of the vacuum when applied to the diaphragms. The diaphragms are mechanically interconnected by a stem which in turn is operatively associated with an end of an actuator rod connected at its other end to a damper incorporated in an automotive air conditioner. The actuator is arranged such that, when the valve means is operated to introduce vacuum into one of the two vacuum chambers, one of the diaphragms is moved or displaced a distance to move the actuator rod the same distance thereby to move the damper to a first stage position and, when the valve means is further operated to introduce vacuum into both vacuum chambers, the other diaphragm is displaced to move the damper to a second stage position.
The two-stage actuator discussed above, however, has shortcomings that it requires a large number of component parts; namely, two diaphragms of an elastomeric material, stem and actuator rod connected to the diaphragms, housing members of sheet metal sealingly secured to the outer peripheral edges of the diaphragms and springs installed in the vacuum chambers, that complicated steps are required to assemble these members together into the actuator resulting in an increased cost of manufacture and that a strict quality control is required to eliminate leakage which would otherwise occur if the components of the actuator are not completely sealingly assembled. An additional shortcoming is that, because the diaphragms are secured at their outer peripheries to the housing, the diaphragms can be deformed limited distances resulting in relatively short actuating strokes.